Vacuum processing is commonly used in the manufacture of semiconductor devices and flat panel displays to deposit thin films on to substrates, and in metallurgical processes. Pumping systems used to evacuate relatively large process chambers to the desired pressure generally comprise at least one booster pump connected in series with at least one backing pump.
Booster pumps typically have oil-free pumping mechanisms, as any lubricants present in the pumping mechanism could cause contamination of the clean environment in which the vacuum processing is performed. Such “dry” vacuum pumps are commonly single or multi-stage positive displacement pumps employing inter-meshing rotors in the pumping mechanism. The rotors may have the same type of profile in each stage or the profile may change from stage to stage. The backing pumps may have either a similar pumping mechanism to the booster pumps, or a different pumping mechanism.
An asynchronous AC motor typically drives the pumping mechanism of a booster pump. Such motors must have a rating such that the pump is able to supply adequate compression of the pumped gas between the pump inlet and outlet, and such that the pumping speed resulting is sufficient for the duty required.
A proportion of the power supplied to the motor of the booster pump produces heat of compression in the exhaust gas, particularly at intermediate and high inlet pressure levels, such that the pump body and rotors can heat up. If the amount of compression and differential pressure generated is not adequately controlled, there may be a risk of overheating the booster pump, ultimately resulting in lubrication failure, excessive thermal expansion and seizure. The standard motor for the size and pumping speed of the booster pump is thus usually selected such that it should be able to supply adequate compression in normal use at low inlet pressures but a risk of overheating remains if the pump is operated at intermediate and high inlet pressure levels without a means of protection.
In the conventional pumping system described above, frequent and repeated operation at high to intermediate inlet pressures may be required. The amount of gas compression produced by the booster pump, and the differential pressure generated between its inlet and outlet, may be limited by various means to control the amount of heat generated and to limit the risk of overheating. If the gas compression produced by the booster pump is limited too severely, the resulting evacuation time of the large vacuum chamber may be undesirably slow. If the gas compression produced by the booster pump is not limited enough, whilst the resulting evacuation time of the vacuum chamber may be rapid the mechanical booster pump may overheat.
For driving the motor of a booster pump, a variable frequency drive unit may be provided between the motor and a power source for the motor. Such drive units may operate by converting the AC power supplied by the power source into a DC power, and then converting the DC power into an AC power of desired amplitude and frequency. The power supplied to the motor is controlled by controlling the current supplied to the motor, which in turn is controlled by adjusting the frequency and/or amplitude of the voltage in the motor. The current supplied to the motor determines the amount of torque produced in the motor, and thus determines the torque available to rotate the pumping mechanism. The frequency of the power determines the speed of rotation of the pumping mechanism. By varying the frequency of the power, the booster pump can maintain a constant system pressure even under conditions where the gas load may vary substantially.
The drive unit sets a maximum value for the frequency of the power (fmax), and a maximum value for the current supplied to the motor (Imax). This current limit will conventionally be appropriate to the continuous rating of the motor, and will limit the effective torque produced by the pumping mechanism and hence the amount of differential pressure resulting. This maximum current is typically the current that can be sustained indefinitely without overheating the motor.
At the start of a rapid evacuation cycle, it is desirable to rotate the pumping mechanism as rapidly as possible to maximise the evacuation rate. Due to the high pressure, and thus relatively high density, of the gas at the start of the cycle, a large torque is required to initiate rotation of the pumping mechanism at a frequency around fmax, and so there is a high current demand. The torque required to rotate the pumping mechanism may be increased when the pump is used to evacuate a process chamber in which particulates are generated as a by-product of the processing performed in the chamber. These particulates can accumulate within the pump and effectively fill the vacant running clearance between the rotor and stator components of the pumping mechanism. When the pump is stopped the rotor and stator components will cool and shrink. Due to the different thermal expansions of the rotor and stator components of the pump, the running clearances between the rotor and stator components reduce. However, if those running clearances are already full of particulates, then those particulates become crushed between the rotor and stator components, and can effectively apply a brake to the rotor components so that, in severe cases, the torque that can be produced by the supply of a current of Imax from the drive unit to the motor is insufficient to re-start the pumping mechanism.
Whilst variable frequency drive units may be provided with an overload capability, the overload capability is usually around 150% of the rated power for a short term, time limited period, and so even when operated in an overload condition it may not be possible to re-start the pumping mechanism.
Despite this, an advantage associated with the overload capability of a variable frequency drive unit is that the motor can be deliberately operated in overload for a short period of time in order to reduce the time required to evacuate the chamber from atmospheric pressure to the desired low pressure (the “pump down” time). However, in the event that the overload time period is inadvertently exceeded, the frequency of the power supplied to the motor of the booster pump will be rapidly reduced to some level below fmax to protect the motor from damage, resulting in a sharp reduction in the rotational speed of the pump while limiting the differential pressure produced. As the evacuation progresses and the inlet pressure decreases, the drive unit will ramp up the frequency towards fmax over a finite period to gradually increase the rotational speed of the booster pump. While this protects the booster pump from overheating at all inlet pressures, this period when the rotational speed is reduced may represent an undesirable extension of the pump down time.